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Figure 5. 10.000 animated characters in test VE
Name Processor Memory | Graphics Hardware
Test Intel Pentium 4,1256 mb | NVidia GeForce2
PC#1 1.8 Mhz go 400, 32 mb
Test Intel Pentium 4,| 512 mb | NVidia GeForce4
PCA2 2.4 Mhz go, 460 64 mb
Test Intel Pentium 4,|2 gb NVidia GeForceFX
| PC#3 3.2 Mhz 5900, 128 mb
Table 1. Configuration of Test PCs
Test PC # of Soldiers in Viewing Frustum Frame
Rate
Test PC#1 3000 4 fps
Test PC#1 380 20 fps
Test PC#1 110 30 fps
Test PC#1 1100 10 fps
Test PC#2 300 30 fps
Test PC#2 800 15 fps
Test PC#2 1970 6 fps
Test PC#3 1700 20 fps
Table 2. Rendering performance results
2.3 Sound
Let us imagine a scenario that happens in immersive VR
system: The user walks in the geo-specific VE. When he wants
to learn the name of the hill he faces, one way is to display text
that shows the name. Although the user is informed, the
displayed text destroys realism, as in the real world we do not
see geographic names on the hills. Also this solution is not
different than using traditional 2D GIS or paper map. Another
solution, which we prefer, is text to speech mechanism. By
using GIS import tool we get the coordinates of geographic
locations in Gazetteer and defined a buffer around them. For
example when the user touches a hill with data glove he hears
the name of the hill. We simulated this scenario on desktop VR
with mouse and evaluated it useful. Regarding the use of sound
in VE we used library of wav files that contains sound effects
such as wind, marching group, rain, various engine sounds etc.
Although these simple sounds contributed the realism of the
environment more realistic use of sound is essential. Real world
effects such as Doppler effect may better impress the user.
2.4 Conceptual Elements
2.4.1 Sky and Clouds: Atmospheric rendering is an
important step to generate impressive virtual environments.
There are a lot of methods for sky and atmosphere rendering,
ranging from the use of single color to very realistic models.
The sky color is time and location dependent. It is a known fact
that the sky color around the horizon is not same with the sky
color around zenith at the same time. It is also known that the
sky color around horizon becomes red at sunrise and sunset.
The altitude of the sun, the viewing direction, the height of the
observer, conditions of the atmosphere, and the reflected light
from the ground are the parameters that affect the color of the
sky (Nishita et al., 1996). It is a very complex task to try to
render sky according to criteria listed above. In order to
simplify this complicated task we built pre-rendered skybox
library. Skybox is a cube in which inner faces are texture
mapped with five or six pre-rendered images. When this cube is
folded, inner faces create a seamless scene. Aesthetic skyboxes
need artist work. The easy way to create them is to use special
landscape rendering packages. It is also possible to obtain
skybox image sets on Internet. Our skybox library contains
many consequent scenes that complete a day loop.
Figure 6. Skybox
2.4.2 The Sun and the Moon: VR applications that render
real world conditions use the Sun and the Moon as light sources
and complementary objects of the VE. In both usages, it is
important to place them into their correct positions in the three-
dimensional scene. To calculate the positions of the Sun and the
Moon at a given time and location, some methods use
astronomic almanacs and complex equations, which give
precise results and some others use simple formulas to get
rough results. Jean Meeus, a Belgian astronomer published a
book Astronomical Algorithms for computer calculations, which
became popular among amateur astronomers and computer
programmers (Meeus, 1991). Geocentric positions are accurate
to within a few arc-seconds, which is many times higher than
typical desktop display resolution precision. In order to
correctly visualize the Sun and the Moon it is necessary to
calculate angular sizes and locate them on the outer border of
the limited VE.
© — 2arctan(r / d) (5)
where . O- angular size of celestial body
r= radius
d= distance to the Earth.
Rendering of the Moon is quite different since it necessary to
determine the visible portion and the bright limb angle, which
corresponds to inclination with respect to rotation axis.
